Ink jet printing has become a prominent contender in the digital output arena because of its non-impact, low-noise characteristics, and its compatibility with plain paper. Ink jet printing avoids the complications of toner transfers and fixing as in electrophotography and the pressure contact at the printing interface as in thermal resistive printing technologies. Ink jet printing mechanisms includes continuous ink jet or drop-on-demand ink jet. U.S. Pat. No. 3,946,398, which issued to Kyser et al. in 1970, discloses a drop-on-demand ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Piezoelectric ink jet printers can also utilize piezoelectric crystals in push mode, shear mode, and squeeze mode. EP 827 833 A2 and WO 98/08687 disclose a piezoelectric ink jet print apparatus with reduced crosstalk between channels, improved ink protection, and capability of ejecting variable ink drop size.
U.S. Pat. No. 4,723,129, issued to Endo, discloses an electrothermal drop-on-demand ink jet printer wherein a power pulse is applied to an electrothermal heater which is in thermal contact with water based ink in a nozzle. The heat from the electrothermal heater can produce a vapor bubble in the ink, which causes an ink drop to be ejected from a small aperture along the edge of the heater substrate. This technology is known as Bubblejet.TM. (trademark of Canon K.K. of Japan).
U.S. Pat. No. 4,460,728, which issued to Vaught et al. in 1982, discloses an electrothermal drop ejection system which also operates by bubble formation to eject drops in a direction normal to the plane of the heater substrate. As used herein, the term "thermal ink jet" refers to both this system and the system commonly known as Bubblejet.TM..
Ink nozzles are an essential component of an ink jet printer, arrays of nozzles being typically provided in an in ink jet nozzle plate. The shapes and dimensions of the ink nozzles strongly affect the properties of the ink drops ejected. For example, it is well known in the art that if the diameter of the ink nozzle opening deviates from the desired size, both the ink drop volume and the velocity can vary from the desired values. In another example, if the opening of an ink nozzle is formed with an irregular shape, the trajectory of the ejected ink drop from that ink nozzle can also deviate from the desired direction (usually normal to the plane of the ink jet nozzle plate).
Some known methods of forming ink jet nozzle plates use one or more intermediate molds. One such method uses an electroforming process. The electroforming process uses a mold (or mandrel) overcoated with a continuous conductive film having non-conductive structures that protrude over the conductive film. A metallic ink jet nozzle plate is formed using such a mold (or mandrel) by electroplating onto the conductive film. Over time, the metallic layer grows in thickness. The ink nozzles are defined by the non-conductive structures. One difficulty associated with the above method is the need for the intermediate molds or mandrels. The intermediate molds increase the number of steps in the fabrication process. It is well known in the field of micromachining, that the manufacturing variability increases with the number of the steps in the fabrication process. Since the ink jet nozzle plate comprises structures of small and critical dimensions, it is highly desirable to develop a fabrication process that has fewer number of fabrication steps and does not require the use of intermediate molds or mandrels.
A further need for ink jet nozzles in an ink jet printing apparatus is optimization of the nozzle shape. It is well known in the art that the inside surfaces of an ink nozzle can exist in cone, cylindrical, or toroidal shapes with the axis of symmetry generally in the direction of drop ejection. Furthermore, the ink nozzle cross-section perpendicular to the direction of drop ejection can be circular, square or triangular. The structural designs of the ink nozzles can strongly affect the dynamics of the ink fluid during ink drop ejection and refill and therefore determine to a large extent the properties of the ejected ink drops.